Polyolefin-based resin foams are light weight and exhibit an excellent heat resistance and impact resistance, and as a result are widely used in applications such as core materials for automotive interior trim, building materials, stationery, food containers, and so forth.
These polyolefin-based resin foams are obtained, for example, by mixing any of various foaming agents under the application of pressure into a molten polyolefin using an extruder and then carrying out extrusion and foaming under atmospheric pressure from a die attached at the end of the extruder.
However, a problem with conventional foams has been the difficulty of maintaining, in the molten resin, the individual bubbles (also referred to below as “cells”) having the produced gas partial pressure, which has resulted in the rupture of the individual cells and a pronounced tendency for open cells to be produced.
Various means for raising the melt tension of the resin used and thereby raising the cell retention force have been proposed as methods for obtaining a foam with few open cells (excellent closed cell characteristic).
For example, methods have been proposed in which the melt tension is raised through the addition of an ultra-high molecular weight component. However, generally when a resin having a high melt tension is used, the cell retention force is in fact raised, but the viscosity also becomes too high and an expansion ratio that corresponds to the amount of foaming agent addition is not obtained. Moreover, there is a large load on the extruder, and, in those instances in which the maintenance of a high productivity is sought, the load on the extruder is increased and the extrusion moldability declines and, in addition, the temperature of the resin rises due to shear heat generation and cell growth cannot be inhibited through cooling and open cells are produced.
In addition, polypropylene-based resins that are characterized by a high melt tension are commercially available in the form of, for example, propylene-based resins that have been subjected to electron beam crosslinking, as in, e.g., Patent Document 1, and propylene-based resins that have been crosslinked using, e.g., a peroxide, as in, e.g., Patent Documents 2, 3, and 4. However, the use of a crosslinking treatment to raise the melt tension has led to a substantial decline in the melt properties when the edge from foam sheet molding or the excess after container molding is returned again to foam sheet molding, and disadvantage in cost, and due to the crosslinking, this has also been unsatisfactory with regard to extrusion stability and odor.